Fermentations of 3-oxo-.DELTA..sup.4 -steroids with living cultures of known steroid-.DELTA..sup.1 -dehydrogenating microorganisms, such as Bacillus lentus ATCC 13 805, Bacillus sphaericus ATCC 7054, ATCC 7055, and ATCC 12 488, Bacillus subtilus NRRL B 558, Arthrobacter simplex, as well as Norcardia corallina ATCC 4273 and 4275 frequently produce unsatisfactory results, in part because the conversion rate is two low or no adequate conversion of the substrate is observed, and in part because the substrate is greatly metabolized.